The Graceful Apocalypse: Part 1 - Basic Thunderstorm Knowledge

Tuesday, August 13, 2013
4:25 P.M.

Lightning behind the Space Needle. Photographed last Friday night (8/9 or 8/10). Retrieved from Anthony May Photography's Facebook Page. Author: Anthony May

Last Friday night will rank up, at least in my mind, with some of the most memorable weather events in Seattle's history. That means I'll put it right up there with the Inaugural Day Storm, the Hanukkah Eve Storm (and associated torrential downpour), the December 2008 snow events, the 2007 Great Coastal Gale... you get the idea. Photographer Anthony May took the above picture of lightning behind the Space Needle last Friday night, and it has since gone viral. It will surely withstand the test of time and go down as one of the most incredible pictures of the Seattle skyline ever taken. Anthony's Facebook page www.facebook.com/anthonymayphotography, and his personal website is anthonymayphotography.com. I HIGHLY recommend that you check them out... he takes some stunning pictures.



 I have never seen such a spectacular lightning show in Seattle. Storms like this with heavy lightning and relatively light precipitation are not that uncommon for other parts of the Intermountain West. Events like this had already happened in Eastern Washington this summer. But for Seattle, they are unusual. I do remember a lightning show similar to this that must have happened in my elementary school days, but I cannot remember a show this spectacular, this prolonged, and this dangerous.

Since this event was so awe-inspiring, I am going to blog about it in four parts. The first part will be an introduction to the formation and characteristics of thunderstorms with a focus on explaining lightning. The second part will feature a thorough meteorological analysis of what happened not only Friday night in Seattle but all over the weekend throughout the state. The third part will take a look at the fires, floods, and other damage caused by these storms. The fourth and final part will tie everything together with comparison of how our most recent event compares with others in the state and others around the world.

Without further ado, let's get started. 

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Most of us are familiar, whether we know it or not, with many of the meteorological phenomena we experience here in the Pacific Northwest. Warm fronts and cold fronts, high pressure and low pressure, and heat index and wind chill are just a few of the aspects of weather we experience throughout the year. Most of the stuff I talk about is readily explained. Rain forms when cloud droplets coalesce onto an airborne particle until it gets big enough to fall through the cloud. Snowflakes are the frozen analog to rain droplets and are more complicated due to their crystalline structure, but even they make intuitive sense. The sun is responsible for heating the Earth, and greenhouse gasses are responsible for preventing some of the infrared radiation reflected from Earth from escaping out to space. Lightning forms when an electrical field is created within a cloud by scientifically-debated processes and creates plasma which leads to the creation of electrical step leaders while positive streamers are forming via attraction of the positive charge of Earth's surface to the leaders and are eventually met by the leaders to form a closed circuit that sends a return stroke that flows from positive to negative charge that heats up the air around it to five times the temperature of the surface of the sun and creates a shockwave that can be heard as thunder.

What?

Yeah, lightning is not easy to describe. I might as well split part one into separate parts. Let's start out with some basics and go over how a cumulonimbus cloud is created.

1.1 - Creation and Characteristics of a Cumulonimbus Cloud

Cumulonimbus is a conglomerate of the Latin words cumulus and nimbus. Cumulus means a 'heap or pile' and is used to describe clouds of various shapes and sizes that look like somebody got a whole bunch of water droplets and decided to pile them up into one cloud. Nimbus simply means that rain falls from the cloud. Smack these two words together, and BAM!; you got a heapish cloud that rains. Some people will tell you that cumulonimbus and thunderhead are synonymous, but this is not true. Just because a cloud is a cumulonimbus does not mean that it is producing lightning.

There are four types of cumulus clouds: Humilis, mediocris, congestus, and fractus. Humulis are small and harmless, mediocris are slightly larger, congestus (also known as 'towering cumulus') reach high into the sky, and fractus are cloud fragments that have broken off from other clouds. I think that four different species of cumulus clouds is overkill, so let's just talk about humilis and congestus.

. Cumulus humilis over a golden meadow. Retrieved from Wikimedia Commons, Author: PiccoloNamek

Cumulus congestus over the Cahokia Mounds Museum and Interpretive Center. Photographed Sept 2001. Retrieved from Wikipedia Commons. Author: Carptrash

I like to refer to these clouds as cotton-ball and cauliflower clouds, respectively. Cotton-ball cumulus are an indicator of fair weather. See how the tops do not rise very high? This is because the air is fairly stable and does not allow cloud-producing updrafts to rise very high into the atmosphere. With cauliflower clouds, the atmosphere is unstable and has a high environmental adiabatic lapse rate (it cools sharply with elevation), so parcels of air near the ground are quite buoyant and tend to rise. It's these cauliflower clouds that lead to cumulonimbus clouds.

Below are the three stages of cumulonimbus development: a rising cumulus congestus cloud, a mature cumulonimbus with updrafts and downdrafts, and a dissipating storm with the updraft choked off. We've gone over the towering cumulus/cumulus congestus/cauliflower cumulus clouds, so let's go over the mature cumulonimbus cloud.

Diagram from NOAA National Weather Service training materials showing the formation and dissipation of a thunderstorm. Created 7 Jun 2010. Retrieved from Wikimedia Commons. Author: NOAA

If a cauliflower cumulus keeps getting bigger and bigger, it will turn into a cumulonimbus cloud. There are two types of cumulonimbus clouds: Cumulonimbus calvus and Cumulonimbus capillatus. Calvus have a puffy top and look similar to a towering cumulus cloud, but they are even larger. A mega-cauliflower cloud, if you will. Capillatus have wispy, cirrus-like tops.

Cumulonimbus calvus in Wagga Wagga, Australia. Created 12 Jan 2007. Retrieved from Wikimedia Commons. Author: Bidgee.

Cumulonimbus capillatus have a subtype: Cumulonimbus capillatus incus. The incus means that the wispy tops of the clouds are spread out and bear resemblance to the classic 'anvil' shape that we associate with big thunderstorms. Compare this...

An ad for an Acme anvil in a 1902 Sears Catalog. Retrieved from Wikimedia Commons. Uploaded by Jef poskanzer.

... to this...

Cumulonimbus calvus incus. Uploaded 7 April 2005. Retrieved from Wikimedia Commons. Author: Simon.

This anvil marks the beginning of the tropopause - the region in which the troposphere is transitioning to the stratosphere. Whereas the temperature of air decreases in height in the troposphere, it increases with height in the stratosphere due to heat given off in ozone-forming chemical reactions. Because air becomes colder and thus denser when it rises, it cannot rise very far through the stratosphere as gravity simply pulls it down back to a place where it is just as dense as the atmosphere around it. Capillatus with especially strong updrafts often have what is called an "overshooting top," which is a well-defined 'dome' cloud created by the momentum of the air parcel within the updraft thrusting the cloud into the stratosphere until it can go no further.

Photo of a supercell thunderstorm looking east. The presence of an overshooting top above the anvil suggests a very intense storm with an increased potential of being severe. Retrieved from Louisville, KY NWS Forecast Office. Author: Howard Bluestein, University of Oklahoma

Overshooting tops are rare here in the Northwest, but I have seen them. The last one I can remember seeing was associated with a thunderstorm by Mt. Rainier on July 29, 2009 - the warmest day on record in Seattle. That storm spawned a flash flood warning from the NWS.

Here's a picture with both cumulonimbus calvus and cumulonimbus capillatus. Can you make out the differences?

More cumulonimbus calvus with cumulonimbus capillatus in the background as seen from Milano-Malpensa airport to the north in August 3, 2010. Created 3 Apr 2010. Retrieved from Wikimedia Commons. Author: Ximonic,.

Anyway, let's get back to the characteristics of a mature cumulonimbus cloud.

Diagram of a typical thunderstorm. Retrieved from gotoknow.org.

The towering cumulus cloud was formed solely by updrafts. A mature cumulonimbus has not only an updraft to continue supplying fresh air to the storm but a downdraft through which precipitation in the form of rain, hail, and even snow falls. One would think that the temperature under a downdraft would be warm due to adiabatic warming associated with sinking air, but the temperatures under downdrafts are quite cool, especially when the storm is strong. Remember, it takes an unstable atmosphere with a sharp decrease in temperature with height to form a cumulonimbus cloud, so the tops of these clouds are often exceptionally cold. To make matters even more frigid, the air can be cooled further by hail falling through the downdraft and the sublimation of ice crystals into water vapor.

Because this air is cool, it is more dense than the surrounding atmosphere. In some cases, this characteristic can be deadly. Downbursts are rapidly falling parcels of air that can cause massive destruction over an area they impact. They are very dangerous to airplanes because of the extreme turbulence, and since airplane pilots generally encounter them in the lower atmosphere where they are taking off or preparing to land, the stakes are even higher. After they hit the ground, they spread horizontally in all directions. Often times, the damage in tornadic storms is higher from these straight-line winds than the tornado itself.

Downsburst damages in northwest Monroe County, WI. Photographed 27 July 1998. Retrieved from Wikimedia Commons. Author: Todd Shea, La Crosse National Weather Service Office

Updrafts can also cause extreme winds. A massive thunderstorm producing prodigious amounts of rain and hail needs a lot of energy to support it, so air nearby won't waste any time flowing into the updraft. These intense updrafts can cause tornadoes by shoving rotating air within the storm upright.

In addition to the overshooting top, thunderstorms have other characteristic features. Some have mammatus clouds, which form when ice crystals under the anvil sublimate and cool the air (the transition from ice to gas takes energy). This cold air sinks in little pockets and finally stops when all the ice crystals sublimate. These clouds are often indicative of a strong thunderstorm, as they can only be supported by a very moist updraft.

Mammatus clouds over the city of Regina, Saskatchewan following a severe storm warning and tornado watch. Created 26 June 2012. Retrieved from Wikimedia Commons. Author: Craig Lindsay.

Two arcus clouds, shelf clouds and roll clouds, are also associated with thunderstorms. Arcus clouds are long, low-lying, and horizontal clouds which often form on the leading edge of thunderstorms, although roll clouds can be found detached from any sort of separate cloud or weather system. 

Shelf cloud over Enschede, The Netherlands. Created 17 July 2004. Retrieved from Wikimedia Commons. Author: John Kerstholt

Shelf clouds are ominous, wedge-shaped clouds that form on the leading edge of a thunderstorm. These form because cold air from the downdraft of a storm spreads horizontally when it hits the surface. This outflow of air originating from within the downdraft undercuts and lifts the warm, moist air rising into the storm upwards. Shelf clouds form ahead of the "gust front" of a storm, which is essentially a mini-cold front on the leading edge of strong thunderstorms.

If there is enough wind shear due to differences in wind speed with elevation, the opposite directions of the air going out of and into the thunderstorm will "spin up" a roll cloud just behind the gust front.

A roll cloud over downtown Racine, WI. Taken 1 June 2007. Retrieved from Wikimedia Commons

Put these all together, and you've got a typical thunderstorm! Not all thunderstorms have overshooting tops, mammatus clouds, and arcus clouds... those only occur in strong storms or supercells. Here's an average thunderstorm below.

Photo of a thunderstorm with labeled parts. Retrieved from Weather.gov. Author: UCAR

Cumulonimbus clouds dissipate when the updraft is choked off. If no new air is coming into the storm, the storm cannot support itself. This is usually because the downdraft has overtaken the updraft. The cloud will take on a fuzzy appearance, the wind and rain will lighten, and the skies will clear. Most rain events associated with cumulonimbus clouds, thunderstorm or not, last less than an hour.

Dissipating thunderstorm. Retrieved from Sam Houston State University. Credit: NOAA

We've talked about the different types of clouds. Well, I talked about the different types of clouds. You've tried to follow my unnecessarily long explanations. Let's talk about the different types of thunderstorms.

1.2 - Four Types of Thunderstorms: 

Retrieved from UCAR

There are four different types of thunderstorms: single-cell, multi-cell, squall line, and supercell. Supercell thunderstorms are every weather geek's favorite thunderstorm because they are the ones that can spin up tornadoes.

Single Cell:

Single Cell Thunderstorm Over Florida. Created 14 August 2013. The blue is just background noise in the atmosphere and does not indicate rainfall. Retrieved from NWS National Radar Mosaic.

Single cell thunderstorms are actually quite rare. For a thunderstorm to be classified as a single cell storm, it must have no other cells in the vicinity. These storms are generally harmless and last around a half hour. Some of these storms can be stronger and produce hail, torrential rain, and microbursts, but these are the exception, not the rule, and are still short-lived. Since single-cell storms are so isolated and seem to occur at random times and locations, they are hard to predict, so cut the weatherman a little slack if you get smacked by one of these storms.

Multicell:  
 

Multicell Cluster Over Northwest Nebraska. Created 14 August 2013. Retrieved from NWS National Radar Mosaic.

Multicell clusters are much more common than single cell storms. Instead of one isolated thunderstorm just hanging out in the middle of nowhere, multicell clusters consist of a group of cells moving along. Most are short lived, but since these cells have formed in a cluster, the region in which they formed must have been conducive to thunderstorm formation. As such, individual cells will constantly form up and dissipate, but the cluster as a group will retain the same general characteristics. Multicell storms are usually more potent than single cell storms, but they are just harmless punks compared to supercells.

 Multi-cell Line:

Animated gif of an extremely large squall line over the Great Plains during a storm on June 5, 2008 (time at the bottom is in UTC). Retrieved from Wikimedia Commons. Author: NWS

Multi-cell lines, also called squall lines, consist of a more-or-less continuous long line of storms that form at or ahead of a cold front with a gust front at the leading edge. These storms often have breathtaking shelf clouds and have the heaviest rain and hail just to the west of (behind) the updraft. Squall lines can produce golf ball sized hail, weak tornadoes, and, of course, torrential rain, but they are best known for their powerful downbursts and resulting straight-line winds.

Bow Echo near Goodland, Kansas. Created 14 August 2013. Retrieved from Goodland, Kansas NWS Office.

Some downbursts are so strong that they can accelerate parts of the squall line ahead of others, forming what is called a bow echo, and I was lucky enough to observe one on radar tonight (August 14). In cases with sustained winds over 58 mph, storms that take the form of a bow echo are called derechos and can cause severe damage. I checked the county alerts on the NWS homepage and did not observe any severe thunderstorm warnings associated with this echo, so it is not a derecho, just a squall line taking the form of a bow echo.

Supercell:

Supercells garner the most attention from everybody. They are extremely prominent and beautiful thunderstorms, but underneath that mask of beauty lies a ferocious beast that can cause massive hail, powerful downbursts, and most of all, tornadoes.

Features of a supercell. Note: This is a typical northwestward view in North America. Uploaded 10 Aug. 2005 by Demonburrito. Retrieved from Wikimedia Commons. Author: NOAA.

The anatomy of a supercell is similar to a typical strong thunderstorm. I've reposted the thunderstorm anatomy picture from earlier in the blog above for convenience, and I've posted a similar picture of a supercell thunderstorm below. The difference is that the regular thunderstorm does not rotate, while the supercell does. This is evidenced by a wall cloud and a tornado in the supercell diagram while nothing of that sort exists in a typical thunderstorm.

How does the thunderstorm get rotating? Well, first off, you need wind shear, and lots of it. Wind shear is defined as different winds blowing at different elevations in the atmosphere, so in order to have high wind shear, you need to have a large change in wind speed with height. If you choose your inertial frame of reference to be the air moving along the ground, the velocity of the air above it must be much higher for there to be any sort of wind shear. When you have high wind shear, air tends to rotate within the storm.

The high wind shear is very conducive to storm development and these storms can reach astounding heights. Supercells over 40,000 feet high are not all that uncommon. I remember I was flying on a plane at a cruising altitude of ~37,000 feet and there was a massive thunderstorm that extended well above our elevation (it was hard to judge how much higher the thunderstorm was than the airplane, but I feel like a mile would be a conservative estimate. Wind shear can also act to separate the updrafts and downdrafts in a storm. Because the downdraft cannot drown out the updraft, supercell storms can last for hours. Thunderstorms in the tropics can rise to an unbelievable 80,000 feet, but this is due to intense convection from strong solar heating. There is very little wind shear in the tropics, and these storms are usually pretty short lived.

During the late afternoon and early evening of April 3, 2004, this supercell thunderstorm dropped 2 inch-diameter hail over Chaparral, N.M. causing widespread damage. Retrieved from NOAA. Author: Greg Lundeen.

I just... as in one minute ago... thought of an useful analogy. Think of an air parcel like a treadmill that is turned off. When you get on that treadmill and push  the conveyor belt forward under your own power, you are acting as the strong wind on the top of the air parcel causing it to rotate. Since this same forward force is not being applied to the conveyor belt on the bottom of the treadmill, the conveyor belt goes down, around, and back up, where you continue to provide the horizontal force necessary to keep the belt rotating.


Second, you need very strong updrafts. According to the NWS, updrafts into a supercell are EXTREMELY strong and can reach speeds of 150-175 miles per hour. Here's a hypothetical mathematical scenario: if you think of the storm as being on a grid with the ground being represented by the x and y axes and elevation being the z-axis, the rotating air parcels are centered on the x axis (and are thus parallel with the ground) and rotate in the direction of the y axis. If the updrafts are parallel to the z-axis, they can flip this rotating air parcel so that it is now centered along the z axis (and thus perpendicular to the ground) and rotates in the direction of the x-axis. You have a rotating updraft, or mesocyclone, and therefore a supercell storm.

Severe thunderstorm with mesocyclone 4 miles north of Glasgow, MT. Created 28 July 2010. Retrieved from Wunderground.com. Author: cycloneprone.

Tornadic storms with a powerful mesocyclone often have what is called a "hook echo" on radar. The hook echo provides a good estimate of where the mesocyclone (and the tornado it may spawn) is.

A radar image of the storm that produced the May 3, 1999 Bridge Creek – Moore tornado, an F5 tornado with, at 301 miles per hour, the highest wind speeds ever measured on Earth. Note the hook echo by the mesocyclone. Retrieved from Wikimedia Commons. Author: Storm Prediction Center.

I'd love to provide detailed explanations for how each specific raindrop in a specimen of a typical thunderstorm, but that would be of no use to you and would actually probably be deeply disturbing. I mean, you guys know I like weather and all, but describing every raindrop? They tell you that following your passions will lead you down the road to success, but as I've learned from past experiences, you should always do so with a seatbelt on. So let's talk about lightning.

1.3 - Lightning:

Anthony May's shot again. What a stunning piece of art.

Remember that run on sentence in the first paragraph of this blog? Lightning is far and away the hardest meteorological aspect to explain. I searched for hours looking to find explanations for the formation of lightning, but for each explanation there was, there was another explanation that disagreed with it, often in an arrogant fashion. Finally, I came upon a source that seemed unbiased and actually recognized that scientists do not agree on lightning. I owe much of the following information to HowStuffWorks.com

Let's go over one possible explanation of how lightning forms according to the folks at HowStuffWorks. I feel like they'd have the best idea of how it works.

The Causes of Lightning:

When we have any cloud, we have water droplets in the air. These are not rain droplets; they are much, much smaller, and it takes thousands of cloud droplets to make up one typical-sized rain droplet. These droplets are neutrally charged and do not electromagnetically interact with each other. They interact with each other by colliding with other cloud droplets or precipitation falling from the cloud itself. When these collisions occur, electrons are scraped off the droplets. Because electrons have been knocked off some of the neutral molecules, the molecules are now ions with a net positive charge and rise toward the top of the cloud. The electrons tend to gather toward the base of the cloud

It's not only these collisions that are responsible for creating a giant capacitor of sorts within the cloud. As cloud droplets rise into the upper atmosphere, they cool. But just because they cool below the freezing point does not mean they immediately turn into ice. Some droplets are supercooled, meaning they are liquid at temperatures below freezing. The ice crystals that form are negatively charged, and the supercooled water droplets are positively charged. Updrafts separate the frozen and unfrozen droplets and take the supercooled, positively-charged droplets up to the top of the cloud, while the frozen, negatively-charged droplets travel to the lower portions of the cloud (they may melt on the way down, and I do not know how this melting would affect the charge).

With this charge separation comes an electric field which, like the cloud, is generally negative at the base and positive at the top. This field becomes stronger and stronger as the charges in the cloud become stronger and stronger. In fact, the field can be so strong that the electrons at the base of the cloud can repel the electrons on the surface of Earth into the ground while attracting positive ions upward. Now, you have an "electron sandwich," with positive charges enclosing a region of negative charge. At the very base of the cloud, there is a weak net positive charge, but for the purposes of simplicity, don't give it too much thought. Bottom = negative, top = positive.

Diagram of charges within and around a thunderstorm. Retrieved from and created by the NWS Lightning Safety page.

But what good is this separation if the charges cannot travel to each other? Say, for example, we held the above charge configuration in a rubber medium. Rubber is an insulator and doesn't allow electrons to move. The atmosphere, however, is not. It's not an ideal conductor like copper or silver, but it's not a complete insulator either. By not being an insulator, charges can move and align themselves in the above position. By not being an extremely effective conductor, charges can build up without being constantly transferred from one place to each other. For lightning to form, all we need is for the charge separation to become so great that a conductive path is formed through the air and provides a line through which the charges can freely interact and release excess charge. But how do we form that electric path?

Air Ionization and Plasma:

If the electric field becomes exceptionally strong (tens of thousands of volts per inch), the air itself is separated into electrons and positive ions. When this happens, the air becomes a much better conductor of electricity and is called plasma. We've all learned about three states of matter: solid, liquid, and gaseous, but plasma is a state of matter as well, bumping up the number of states of matter to four.* This process of separation lays out a path for electricity to travel from higher potential to lower potential in the same fashion that a mole makes a tunnel from the neighbor's yard to your yard. It's hard to compare plasma to mammals, but you get the idea.
 
*Technically, there are many other states of matter, both observed and theorized, but these only occur under very specific or extreme situations. I'm a big fan of quark-gluon plasma and Bose–Einstein condensate, but my favorite has to be strongly symmetric matter, a state in which the four fundamental forces of the universe were unified into one grand force. It is theorized that it lasted for all of 10⁻³⁶ seconds after the Big Bang. Don't you just love physics?

Step leaders:

Even though plasma is an excellent conductor of electricity, we don't see a constant exchange of electrons and positive ions from one place to another. This is because there is not a homogeneous plasma field that would allow this. Instead, we have step leaders, which are independent paths of ionized air that stem from the negatively-charged region of the cloud. This happens with all types of lightning, but I think it is best visualized using cloud-to-ground lightning, so let's explain it that way.

Just like people, these leaders come in all different shapes and sizes and the atmosphere is filled with particulate matter that can make the leader more likely to go in one direction than the other. If the base of the cloud and the ground are parallel, the electric flux, which is a scalar quantity that represents the rate of flow of the electric field through an area (and therefore the strength of the field), between the cloud and the ground will be maximized when the area through which the flux is being measured is parallel to the ground. By approximating the direction and magnitude of the flux by using arbitrary 'flux lines,' we can see this since flux lines always radiate perpendicularly from their charge surface and then move in the direction of opposite charge.

Of course, no cloud can be perfectly parallel to the ground. The cloud and ground would have to be to identical and parallel planes, and such an idealized situation is not found in nature. And then there's the whole "the Earth isn't flat" thing, not to mention all the particles in and characteristics of the atmosphere that would interfere with the paths of the flux lines. As such,the flux lines will not follow a path straight from the cloud into the ground. Instead, these flux lines intersect and diverge, creating a non-uniform field. It is this non-uniform field that causes the step leaders to take a path not perpendicular to the surface of the intended target.

As you can see below, there is a weak amount of positive charge at the very base of the cloud. As you can see above, I told you to not worry about it to make things easier on yourself. This charge is not sufficient to neutralize the large buildup of static electricity and the electrons continue to flow to the ground.

Negatively charged channel emerges from the bottom of the cloud. Retrieved from the NWS Lightning Safety page and created by NOAA.

These leaders occur in stages. It may not look like there is a pause between them, but that is just because they are occurring so fast. They develop downward on either side of 200,000 mph, but they would be going closer to 186,000 miles per second (the speed of light in a vacuum) if they went downward in an unfragmented fashion.

Step leaders branching toward the ground in stages. Retrieved from the NWS Lightning Safety page and created by NOAA.

Stepped leaders traveling from the negatively charged cloud to the positively charged ground. Retrieved from the NWS Lightning Safety page and created by NOAA.

Now, you've got leaders going toward the ground via the path of least resistance. These leaders are slightly purple. New leaders may form, but every one stays illuminated until the current has reached the ground.

These leaders are NOT the big lightning strikes we are familiar with. In order for those to occur, the circuit needs to be completed. And that's where positive streamers come in.

Positive Streamers:

Public image consultant Álvaro Gordoa demonstrates handshaking technique at a presentation at Monterrey Institute of Technology and Higher Education, Mexico City. Created 29 Oct. 2012. Retrieved from Wikimedia Commons. Author: Angélica Martínez.

I like to think of the whole leaders/streamers thing as a handshake. As public image consultant Álvaro Gordoa shows, somebody always initiates a handshake, and the person who picks up this signal immediately knows to reach their hand out and complete the gesture. The hand of the initiatior doesn't just keep on truckin' until it smashes into the closed fist of the now unsettled would-be recipient of the handshake. And that's what happens with lightning. The stepped leader doesn't just smash into the ground. It meets with what is called a positive streamer to complete the transfer of electrons from cloud to ground.

Since these leaders are negatively charged and the ground is positively charged, the charges attract each other. The ground manifests this attraction by sending these streamers, also purplish in color, into the atmosphere. Once they have been produced, they do not travel up to meet the leaders; they let the leaders come to them. In other words, the hand receiving the handshake from the initiator lets the initiating hand to all the work.

Once these two acquaintances have met, a path for the current to flow between the ground and the cloud is created, and a huge discharge follows. Positive charge from the ground races up this path toward the thundercloud and is visible as the "return stroke" of lightning that is most visible to us. That's right: the strike we see with our naked eye actually starts on the ground and races back to the cloud. Negative charge does all the hard work trying to find a way to get to the ground, and once a connection is established, positive charge surges back up through the circuit.

Multiple Strikes:

There can be as many as 30-40 additional strikes after the initial return stroke. Remember how the step leaders coming to the ground from the base of the cloud were pointed in all these different directions? Well, once the circuit is completed, the electrons in those leaders flow through the leaders into the path of the initial strike. The leaders are only providing a path for the electrons to flow through, not neutralizing the charges themselves. These secondary strikes can be seen as branches off of the initial strike if they follow different paths or just make the initial strike look longer by taking the same pathway (and anywhere in between). Sometimes, the flash from the main strike will end while secondary strikes are  occurring, and this makes the initial lightning strike flicker like a star or blind you like a strobe depending on your proximity to it.

Thunder:

The return stroke, which can be thought of as a plasma channel for charge to travel through, discharges a tremendous amount of static electricity in a very short period of time and therefore heats the air around it to extremely high temperatures - as high as 50,000 degrees C. This heating causes air radiating from the return stroke to expand in the form of a shock wave that we hear as thunder. Thunder is LOUD... my house just barely missed getting struck by lightning during 'thundersnow' storm (a thunderstorm where it is raining instead of snowing) on December 18, 2008. The family cat has never been the same since.

Paths of Lightning:


The example we used above was of cloud-to-ground lightning, which is the most dangerous. People, trees, and animals don't live in clouds now, do they? There are two other paths that lightning can take: intra-cloud and cloud to cloud. Remember, it's not just the base of the cloud and the ground that are charged. There's a whole bunch of positive charge at the tops of the clouds, and lightning can form either in the same cloud or spread from different clouds due to the interactions between these charges. The process through which the lightning takes these paths has all the plasma and streamers and associated phenomena that we went over in cloud-to-ground lightning. Cloud-to-ground lightning is just easier to visualize.

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A 1901 engraving depicting ball lightning. Retrieved from Wikimedia Commons. Uploader: Lantzy.

To leave you with something to ponder, do yourself a favor and look up "ball lightning." It's a truly fascinating phenomenon. Now, onto part 2!

Charlie

Back to the Status Quo

Tuesday, August 13, 2013
12:28 P.M.

Mr. Krupp on the left, and his alter-ego Captain Underpants on the right. Retrieved from the Captain Underpants Wikia Page.

I don't know how many of you read the Captain Underpants books, but I grew up with them. They were based on the premise of Mr. Krupp, an extraordinarily mean elementary school principal, turning into Captain Underpants at the snap of a finger - a superhero with superhuman strength, the ability to fly, and even 100% cotton-power vision - , and then turning back into his mean self when a bucket of water is dumped on his head.

I think that's a beautiful simile for the weather we have seen lately. However, it may have actually had some truth for me. At the first clap of thunder, I turned into an other-worldly human - one that didn't need sleep, one that was capable of withstanding extremely long periods without blinking, and of course, one that was capable of predicting the frequency of lightning strikes associated with one storm (with a little help from the radar, but don't tell anybody).

At that first clap of thunder, millions of individuals became their own Captain Underpants.' They valiantly fought fires and drove hundreds of miles to see the largest storms in the Pacific Northwest. They stood outside and weathered the elements of wind, rain, and even golfball-sized hail, just to get a glimpse of the incredible lightning show we experienced. Some people went to great lengths to document the storms that will surely sweep the 2014 Academy Awards. And as soon as the weather returned to normal, people became their mechanical, grumpy selves just in time to work their 9-5's for the rest of the week.

We had an unusual pattern that produced all these storms for us, which I will explain in detail in a later blog. Let's take a look at what's happening now.

Valid 05:00 am PDT, Tue 13 Aug 2013: UW 8/13/13 12z WRF-GFS: 500mb Absolute Vorticity, Heights

I wouldn't say that this is our typical summer pattern - usually, the ridge of high pressure that is currently well south of the Aleutians is further eastward, shoving the jet stream northward into Alaska, but it's close enough. As you can see, we've got a trough offshore, and this will get slightly closer and then stall off our coast, bringing some weak rain showers (no lightning, sorry) and cooler temperatures to our region.

For the immediate future, we will actually see highs above normal because this trough is generating some downslope flow off the Cascades, thus giving us clear skies and warm temperatures. Wednesday will feature increasing clouds as the trough comes closer to our coast. The model below shows the amount of infrared radiation (heat) emitted from Earth and atmospheric clouds into space and is analogous to a picture one would obtain from an infrared satellite.

Valid 11:00 pm PDT, Tue 13 Aug 2013 - 18hr Fcst: UW 8/13/13 12z WRF-GFS: Outgoing Longwave Radiation

According to this morning's WRF-GFS run, Seattle looks to pick up a whopping tenth of an inch of rain from this entire storm. The Olympics and central Cascades northward will see higher amounts, but even they are only predicted to pick up a half-inch of rain from this storm. Long story short - it won't be a washout.

Valid 05:00 pm PDT, Fri 16 Aug 2013 - 84hr Fcst: UW 8/13/13 UW 12z WRF-GFS: 4km 72-Hour Precipitation

After this "storm" rolls through, we'll enter a pattern we are quite familiar with... morning clouds followed by afternoon sun. See my previous blog below for an explanation of these events.

I hope you can make the best of a return to normal weather. But please, do us a favor and keep those trowsers on.

All the best,
Charlie

Perseids Meteor Shower

Monday, August 12, 2013
3:33 P.M.

Hello! I know many of you are waiting for a blog about our thunderstorms and all that wonderful stuff over the weekend. I've send out requests to certain photographers to use their pictures on my blog, and I've also sent out emails to some of the top meteorological minds around the area to learn more about this setup and why it happened.

Until then, we've got a meteor shower to go through. The following pictures should give the reader a rough idea of what I'm talking about.

Photo Credit: Dante Alighieri

Photo Credit: DO'Neil

On the left is the Willamette Meteorite at the American Museum of Natural History in New York City. On the right is a shower in a bathtub. Put these two together, and voila!: You've got a meteor shower.

Ok, so it's not quite that simple. Even if the plumbing throughout a region could allow meteorites to be used as showering material, I doubt one would actually want to take a shower in them. At the very least, these rocks could annoy you, and in extreme cases, such as the meteorite above, they could crush you. Meteor showers occur when meteoroids that have broken off from comets or asteroids enter the Earth's atmosphere at an angle roughly parallel to the surface of Earth. Most of these meteoroids can only be seen with a microscope, and they disintegrate completely as they encounter friction and ram pressure in the Earth's atmosphere, leaving donkeys, dolphins, dinoflagellates, and all of Earth's wonderful creatures intact. Of course, there is the occasional exception... search "Chicxulub" and you'll learn what I am talking about. 

Before we go any further, let's define the three similar-sounding terms I've thrown at you. Meteors, meteorites, and meteoroids are not the same thing.

Meteoroids: Most of us know what an asteroid is. According to Merriam-Webster, it is a rock going through space that you hope doesn't enter the Earth's atmosphere. In all seriousness, an asteroid is essentially a small planet. It does not have an icy core, and most of them are not spherical, though there are some exceptions. Ceres, the first and largest asteroid ever found, looks nearly spherical. Vesta, the second most massive asteroid, isn't quite as spherical, but it is the brightest asteroid in the solar system and the one we have been able to get the best photographs of.

1 Ceres. Photo Credit: NASA

4 Vespa. Photo credit: NASA

Meteoroids are basically smaller versions of asteroids. They are usually parts of asteroids or comets that have broken off, and but some can be fragments of planets that have been ejected into space by a bolide impact. On average, 33 million pounds of meteoroid matter enters Earth's atmosphere every year, so we are being bombarded by them all year long... some times more than others.

Meteors: Meteors are simply the flashes of light you see when a meteoroid enters Earth's atmosphere. As I said before, friction and ram pressure cause these meteoroids to disintegrate, and as they do so, they create these "shooting stars" that dart through the atmosphere for a second or so.

Meteorites: Meteorites are meteoroids that have survived the fall through Earth's atmosphere and have actually landed on the planet. I was curious to see if anybody got struck by a meteoroid (that was just billionths of a second before coming a meteorite), so I Googled it and found that it has actually happened. got struck by a meteoroid (that was just billionths of a second before coming a meteorite), so I Googled it and found that it has actually happened. Several people have been hit, but the most well-known one is the Sylacauga meteorite, which hit Ann Hodges on November 30, 1952. A grapefruit-sized fragment of the meteoroid smashed through her roof and hit her while she was sleeping. She survived - I have no idea how.

I'm not writing about these near-death-experiences just for kicks. Every year, Earth passes through a large cloud of meteoroids known as the Perseid Cloud. These meteoroids are from particles that have been ejected from comet Swift-Tuttle, a relatively short-period comet that orbits the sun every 133 years. Every year, Earth passes through this cloud. Perseid meteors streak across the sky from mid-July until late August, but they peak in early-to-mid August. Right now we are in the middle of this particular shower, but us Pacific Northwesterners have not been able to see the show because we've had a much more dramatic lightening show over our heads for the past couple of days. We should be able to get some great views tonight as skies will be clear.

2010 Perseid meteor over ESO’s Paranal Observatory in Chile. Photo Credit: ESO

The meteors will peak between midnight and 4 a.m. tomorrow morning with approximately 60 meteors per hour. Enjoy the show!

Charlie

More Thunderstorms Tonight

Saturday, August 10, 2013
3:49 P.M.

Good afternoon everybody! Hopefully most of you experienced that absolutely incredible lightning show last night. Thankfully, if you missed it, we've got another one stepping up to the plate.

First off, let's look at the radar.

Thunderstorms are currently firing up across the area, with some particularly strong ones slightly east of Mt. Rainier, due east of Salem in the Cascades, and slightly west of Baker City, Oregon. I can see the wispy tops of the cell by Mt. Rainer extending northwestward just shy of my Seattle home. The area of heaviest precipitation may be small, but all these storms are massive in size. The precipitation shows up more clearly on the two cells in Oregon. There are light green echos surrounding a red dot in the middle. This red dot indicates the heaviest precipitation, and the light green indicates lighter precipitation that is likely falling from a higher elevation under the anvil of the storm. In this picture, the Mt. Rainier cell does not seem to have these green echoes, but in the minutes since I've taken this screenshot, I have seen these green echoes forming around it.

How well did the models forecast these storms? Honestly, I think they underdid it.

Valid 05:00 pm PDT, Sat 10 Aug 2013 - 12hr Fcst: UW 8/10/13 WRF-GFS: 4km 1-hour Precipitation

Take a look at the model above. You can make out those three individual cells, but they are all weaker than what is currently being shown on the radar, particularly the Mt. Rainier cell. They also do not have any lighter rain surrounding the cells, but I expect that this may be due to the models taking the evaporative affect of dry air on rain into account so that no rain hits the surface. Salem is indeed dry right now with a relative humidity of 23%, so the rain being captured by the radar is evaporating before it hits the surface.

Here's what the National Weather Service office in Seattle had to say in their 4 P.M. AFD (Area Forecast Discussion)

EXPECT 
  ANOTHER ROUND OF THUNDERSTORMS TODAY...ESPECIALLY THIS AFTERNOON AND
  EVENING AS THE LOW CONTINUES ITS SLOW PROGRESSION NORTH ADDING UPPER  
  LEVEL SUPPORT TO THE MOIST AND UNSTABLE CONDITIONS ALREADY IN PLACE.

EXPECT THIS TREND TO CONTINUE THROUGH THE EVENING AS SCATTERED  
  THUNDERSTORMS CONTINUE TO DEVELOP OVER WESTERN WASHINGTON. THESE  
  STORMS MAY PRODUCE SMALL HAIL AND WIND GUSTS TO NEAR 40 MPH.

But according to the 12z WRF-GFS this morning, we won't just see thunderstorms in the afternoon and evening. Just like last night, a shortwave trough is expected to roll through and bring some additional storms to the area. This trough will likely be an hour or two ahead of the trough we saw last night.

Valid 12:00 am PDT, Sun 11 Aug 2013 - 19hr Fcst: UW 8/10/13 12z WRF-GFS: 4km 1-hour Precipitation

Here's the radar right now. More storms are continuing to fire up. Looks like it will be an exciting evening. :)

Charlie

Marine Pushes and Visibility

Friday, August 9, 2013
3:20 P.M.

In many parts of the country, air conditioning is necessary during the summer months. Even in places with high and low temperatures similar to us (usually places that are close to the Canadian border, such as International Falls, Minnesota and Portland, Maine) generally have higher relative humidities. And Chicago? Fu'gett about it. And Texas? Fer'get bout it even more.

For that, I am extremely thankful. I get hot pretty easily, especially after the sun sets, and it's very difficult, nay, impossible for me to sleep when I am overheated. You can always put on more clothes, but you can't keep on taking them off. I generally take a cold shower right before bed so I can cool myself off. With a little meditation and relaxation, you can stay calm while the unheated tap water seeps down your back, and when it's time to get out, you feel oh so good.

This is abnormal. I get pretty hot at night, but most people don't need the whole cold shower ritual. All people, myself included, benefit from what is called a "marine push" or "onshore flow." In a typical Western Washington marine push event, cool air off the upwelled coastal waters of the Pacific flows into the lowlands through the Chehalis Gap and Strait of Juan de Fuca. This relatively cool and moist air brings cooler temperatures and clouds during the morning

But why do we even get an onshore flow? It's not just Seattle - much of the West Coast experiences a similar phenomenon of cloudy mornings and sunny afternoons. San Francisco comes to mind.

Golden Gate Bridge on May 3, 2009 at 7:33 P.M. - Picture by Wikimedia user runner310

However, each time I have woken up after an onshore flow event, I've noticed that the sky is initially hazy. This haze lessens as the sun burns off the stratus clouds in the morning, and it continues to burn off as the temperature rises into the afternoon. I've done some research and I think I have arrived at a correct conclusion. Before I spill the beans and give you the answer, though, let's go over what an onshore flow or marine push event is and how it forms.

_____________________________________________________________

It is first important to understand the correlation between temperature and pressure. Let's start with one of the fundamental rules of physics. When gasses warm, their volume expands while their mass remains constant. For our purposes, a parcel of air rises because it is less dense than the surrounding atmosphere. This is something everybody is familiar with. Does the picture below ring a bell?

Mass Ascension from the International Albuquerque Ballon Fiesta. Taken October 13, 2006 by Eric Ward and retrieved from Wikimedia Commons.

Let's think about it in mathematical terms. We know that density is equal to mass/volume, so let's set up the equation d=m/v. In this equation, v (volume) is the denominator, and as the denominator of a fraction increases while the numerator stays constant, the fraction as a whole decreases in value. 1/2, 1/4, 1/8, 1/999999... you get the point. If we let m = 1 and v = 1, then d = 1. But if we let m = 1 and v = 999999, d is approximately 0.000001. The ratio of the old density to the new density is 1/0.000001 = 1,000,000 (one million). So the old density is around 1,000,000 million times greater than the new density. We usually don't see differences in density that are that large, but you get the point.

The reason hot air balloons work is because they heat up the surrounding air inside their envelope until the entire person/basket/envelope system is less dense than the air around it, and it can rise. Hot air balloons are the oldest type of technology capable of carrying humans through the atmosphere, and as you can see above, they are still alive and well.

Now that we've established the relationship between density and heat, let's look at some of the processes that cause marine pushes to occur.

__________________________________________________

In order to have a marine push, we need to have some marine stuff that is capable of being pushed. Thankfully, we have plenty of marine stuff, and that marine stuff is in the form of cool, moist air over the Pacific Ocean.

Still, why doesn't Hawai'i get marine pushes? Why doesn't Boston? They both have plenty of marine stuff, right?

Recall that an air parcel rises when it is hotter (and therefore less dense) than the surrounding environment. I'm going to show you some pictures I got off a program called Terrafin. Terrafin is a program that is aimed toward helping fishermen, particularly those targeting pelagic species, find water conditions that would be the most favorable for catching certain types of fish. Terrafin provides sea-surface-temperature, chlorophyll, geostrophic current velocity and dynamic height, and sea-surface-height anomaly charts to those with an account. 

Although I only use it for fishing once a year when my family goes tuna hunting off Westport, it is an extraordinary program and provides a great hi-resolution look into what is happening out in the ocean. I highly recommend buying a year-long subscription. Plus, if you say I referred you, I get two months free. Frankly, that's the main reason you should sign up.

Let's take a look at the sea-surface temperatures off Hawai'i and Bahston.

The lowest it gets off Kona is 75 degrees, but most of the stuff is over 80 degrees, which is the same temperature as the land on that side. The absolute lowest it gets off this section of the East Coast is 60 degrees, but temperatures near Boston are 65 degrees. The average high this time of the year for Boston is around 80, so this isn't a huge temperature discrepancy.
Now let's take a look at Washington and Northern California sea-surface temperatures. Some parts are not shown because clouds were covering the area at the time of the shot, and these types of satellites cannot approximate the water temperature if clouds are covering the surface.

Much colder water is near the coast. Off Washington, the water is around 50 degrees. Off Brookings (which is actually in extreme southwest Oregon), the water is even colder: 44 degrees in spots. In both locations, there is a massive temperature difference between the water immediately off the coast and the temperatures of the land adjacent to the coast. Since cold air is denser than warm air, a localized dome of high pressure remains on the coast while the pressure drops over the inland areas each day as the air warms and becomes less dense. This creates a strong pressure gradient from high pressure over the coast to low pressure over land during the day, and this creates a very strong seabreeze. At night, the temperatures are around the same, so no breeze occurs. On another note, some places actually experience land breezes, in which the wind travels offshore because the water offshore is warmer than the temperatures on land. This generally occurs at night. The Pacific Northwest doesn't get much of a land breeze because at night there is not that much of a temperature discrepancy between the land and the ocean.

Often times, these breezes leave evidence that they have occurred, even when they are not actually occurring. Take a look at this picture of a windswept tree off the coast of northern California. Since the pressure gradients off northern California and southern Oregon are very strong due to the strong temperature differences off the coast and onshore in the summer, the area is currently being investigated for the potential for offshore wind farms. There are already plenty of wind farms onshore in that area.

Photo Credit: Ted Goth - April 2010

Now, our feature presentation... the marine pushes that are unique to Western Washington.

First, we have the setup I've already explained. Cold air over the ocean gives a localized area of high pressure, while daytime heating creates lower pressure inland.

Because the Pacific Ocean is so cool, warm air that flows over it causes fog to form as it cools and can hold less invisible water vapor. Meanwhile, the inland areas are super sunny. Often, the fog will burn off in the morning and lead to sunnier conditions in the afternoon due to daytime heating along the coast, and these sunny conditions often translate to a pretty hefty sea breeze, particularly along the Strait of Juan de Fuca.

Due to our sea breeze, all this moist air blows into Western Washington through the Chehalis Gap and the Strait of Juan de Fuca.

As the sun sets and temperatures over Western Washington drop, this moist air off the ocean condenses into stratiform clouds over the lowlands, giving us our classic cloudy mornings.

* All these pictures were based off of ones from a KOMO article on the same thing, which you can find here: http://www.komonews.com/weather/faq/4306832.html. Tanner Petersen, CEO of WeatherOn, did the graphics for me. Send him a thank you by spreading the word about WeatherOn.

Portland actually does NOT get these marine pushes because there is no passage for air to flow through. Therefore, highs are generally 5-10 degrees warmer in the summer there than in Seattle. Marine pushes are not only a function of cold water and warm land... there has to be a pathway for the air to flow through.

My original stipulation for writing this blog was my observation that the visibility was worse in the morning and got better as the day went on. I could not explain this... the amount of particulate matter in the atmosphere didn't change as the day went on, so why would visibility change? Well, I did some research, and figured it out.

When the relative humidity is high, water is more likely to bond to particulate matter in the atmosphere. When water bonds to particles, it makes them bigger, thus resulting in lower visibility. As the day goes on, the relative humidity decreases due to daytime heating (although the dewpoint stays the same, which means there is the same amount of water vapor in the atmosphere), and less water bonds to particulate matter. Therefore, visibility increases!

I could have just written those last two paragraphs, but I decided to give you the whole spiel. There's no point in me explaining the finer points of a concept if you don't have the basics down!

Thanks for reading, and enjoy the lightning tonight!
Charlie